Analysis of cytosolic heteroglycans from leaves of transgenic potato (Solanum tuberosum L.) plants that under- or overexpress the Pho 2 phosphorylase isozyme

During starch degradation, chloroplasts export neutral sugars into the cytosol where they appear to enter a complex glycan metabolism. Interactions between glycans and glucosyl transferases residing in the cytosol were studied by analyzing transgenic potato (Solanum tuberosum L.) plants that possess either decreased or elevated levels of the cytosolic (Pho 2) phosphorylase isoform. Water-soluble heteroglycans (SHGs) were isolated from these plants and were characterized. SHG contains, as major constituents, arabinose, rhamnose, galactose and glucose. Non-aqueous fractionation combined with other separation techniques revealed a distinct pool of the SHG that is located in the cytosol. Under in vitro conditions, the cytosolic heteroglycans act as glucosyl acceptor selectively for Pho 2. Acceptor sites were characterized by a specific hydrolytic degradation following the Pho 2-catalyzed glucosyl transfer. The size distribution of the cytosolic SHG increased during the dark period, indicating a distinct metabolic activity related to net starch degradation. Antisense inhibition of Pho 2 resulted in increased glucosyl and rhamnosyl contents of the glycans. Overexpression of Pho 2 decreased the content of both residues. Compared with the wild type, in both types of transgenic plants the size of the cytosolic glycans was increased.     

Characterization of plastidial starch phosphorylase in Triticum aestivum L. endosperm

Starch phosphorylase (Pho) catalyses the reversible transfer of glucosyl units from glucose1-phosphate to the non-reducing end of an α-1,4-linked glucan chain. Two major isoforms of Pho exist in the plastid (Pho1) and cytosol (Pho2). In this paper it is proposed that Pho1 may play an important role in recycling glucosyl units from malto-oligosaccharides back into starch synthesis in the developing wheat endosperm. Pho activity was observed in highly purified amyloplast extracts prepared from developing wheat endosperms, representing the first direct evidence of plastidial Pho activity in this tissue. A full-length cDNA clone encoding a plastidial Pho isoform, designated TaPho1, was also isolated from a wheat endosperm cDNA library. The TaPho1 protein and Pho1 enzyme activity levels were shown to increase throughout the period of starch synthesis. These observations add to the growing body of evidence which indicates that this enzyme class has a role in starch synthesis in wheat endosperm and indeed all starch storing tissues.     

Girdling affects carbohydrate-related gene expression in leaves,bark and roots of alternate-bearing citrus trees

Effects of girdling on carbohydrate status and carbohydrate-related gene expression in citrus trees were investigated. Alternate-bearing 'Murcott' (a Citrus reticulata hybrid of unknown origin) trees were girdled during autumn (25 Sep. 2001) and examined 10 weeks later. Girdling brought about carbohydrate (soluble sugar and starch) accumulation in leaves and shoot bark above the girdle, in trees during their fruitless, 'off' year. Trees during their heavy fruit load, 'on' year did not accumulate carbohydrates above the girdle due to the high demand for carbohydrates by the developing fruit. Girdling caused a strong decline in soluble sugar and starch concentrations in organs below the girdle (roots), in both 'on' and 'off' trees. Expression of STPH-L and STPH-H (two isoforms of starch phosphorylase), Agps (ADP-glucose pyrophosphorylase, small subunit), AATP (plastidic ADP/ATP transporter), PGM-C (phosphoglucomutase) and CitSuS1 (sucrose synthase), all of which are associated with starch accumulation, was studied. It was found that gene expression is related to starch accumulation in all 'off' tree organs. RNA levels of all the genes examined were high in leaves and bark that accumulated high concentrations of starch, and low in roots with declining starch concentrations. It may be hypothesized that changes in specific sugars signal the up- and down-regulation of genes involved in starch synthesis.     

Phenylalanine ammonia-lyase in potato (Solanum tuberosum L.). Genomic complexity, structural comparison of two selected genes and modes of expression.

Potato (Solanum tuberosum L. cv. Datura) contains approximately 40-50 phenylalanine ammonia-lyase (PAL) genes/haploid genome. Considerable cDNA heterogeneity indicates that at least about 10, and probably more, of these genes are potentially active. One subfamily, represented by one selected member (PAL-1), was analyzed with respect to genomic complexity, nucleotide and deduced amino acid sequence, and mode of constitutive or induced expression. For comparison, a second gene (PAL-2), representing several subfamilies that are easily distinguished from PAL-1, was included in these studies. Extensive structural similarities were observed both between the TATA-proximal portions of the PAL-1 and PAL-2 promoters, particularly in the areas containing putative cis-acting elements, and among all presently known PAL proteins from various higher and lower plants. The relative abundance of PAL mRNA varied greatly in several major potato organs. However, the patterns obtained with probes detecting either total PAL mRNA or more specifically, PAL-1-related or PAL-2-related mRNA species, were the same within experimental error. Mature leaves contained particularly low levels of PAL mRNA. Infection of these leaves with the pathogenic fungus, Phytophthora infestans, resulted in a large, transient induction of PAL mRNA. The relative timing of PAL-1 and PAL-2 mRNA expression, however, differed in compatible (fungus virulent, plant susceptible) but not in incompatible interactions (fungus avirulent, plant resistant). Wounding of leaves caused an extremely rapid and transient induction of both PAL mRNA species.     

Quantitative trait loci for polyamine content in an RFLP-mapped potato population and their relationship to tuberization

DNA-based genetic markers are now widely used by geneticists to locate genes for quantitative traits, and may also serve as a valuable tool for dissecting complex physiological phenomena. Van den Berg et al. (1996a QTL analysis of potato tuberization. Theor Appl Gen 93: 307–316), using restriction fragment length polymorphism (RFLP)-mapped populations of potato, detected eleven quantitative trait loci (QTLs) for tuberization. Taylor et al. (1992 Expression and sequence analysis of cDNAs induced during the early stages of tuberisation in different organs of the potato plant [ Solanum tuberosum L.]. Plant Mol Biol 20: 641–651) have identified one of the genes associated with tuberization as that for the enzyme S-adenosylmethionine decarboxylase (SAMdc), an enzyme of the polyamine biosynthetic pathway. Chromosomal loci for SAMdc and arginine decarboxylase were established on the potato and tomato chromosomal maps, respectively, by hybridizing cDNA probes for these genes to RFLP digests. The polyamine content of leaves from an RFLP-mapped potato population was analyzed by fluorescence detection following HPLC, with quantitation using an internal standard. The data were analyzed by the 'qGene' statistical program, and QTLs for polyamines were detected on seven chromosomes. At least six QTLs were found for spermine, two for spermidine, and two for putrescine. A spermidine QTL was on chromosome 5 linked to marker TG441 , very close to the place where SAMdc mapped. There was some congruence between QTLs for spermine and those previously detected for tuberization and dormancy, but relationships were not consistent.